class for toBson/fromBson, instances for Maybe

For example, you can now use fromBson of Maybe BsonValue (eg the result of
a lookup) to convert to Maybe a.

You can also give a Maybe a to toBson and for Nothing it will produce
BsonNull and for Just a it will create the appropriate BsonValue for a.

Database/MongoDB/BSON.hs

@@ -261,10 +261,16 @@ putOutterObj bytes = do
 putDataType :: DataType -> Put
 putDataType = putI8 . fromDataType
 
-fromBson :: Convertible BsonValue a => BsonValue -> a
-fromBson = convert
+class BsonConv a b where
+    fromBson :: Convertible a b => a -> b
+    toBson :: Convertible b a => b -> a
 
-toBson :: Convertible a BsonValue => a -> BsonValue
+instance BsonConv BsonValue a where
+    fromBson = convert
+    toBson = convert
+
+instance BsonConv (Maybe BsonValue) (Maybe a) where
+    fromBson = convert
     toBson = convert
 
 unsupportedError :: (Typeable a, Convertible BsonValue a) =>
@@ -343,6 +349,11 @@ instance Convertible Int32 BsonValue where
 instance Convertible Int64 BsonValue where
     safeConvert i = return $ BsonInt64 i
 
+instance (Convertible a BsonValue) =>
+    Convertible (Maybe a) BsonValue where
+        safeConvert Nothing = return BsonNull
+        safeConvert (Just a) = safeConvert a
+
 instance Convertible BsonValue Double where
     safeConvert (BsonDouble d) = return d
     safeConvert (BsonInt32 i) = safeConvert i
@@ -426,3 +437,8 @@ instance Convertible BsonValue Int64 where
     safeConvert (BsonInt32 d) = safeConvert d
     safeConvert (BsonInt64 d) = return d
     safeConvert v = unsupportedError v
+
+instance (Convertible BsonValue a) =>
+    Convertible (Maybe BsonValue) (Maybe a) where
+        safeConvert Nothing = return Nothing
+        safeConvert (Just a) = liftM Just $ safeConvert a